Molecule Cleft and Squares with Binicotinic Bishydrazone Ligand: Crystal Structures, Spectroscopic Properties, and Calculation

Abstract

An attractive carboxamido-containing ligand 2,2′-bipyridyl-3,3′-bis(N,N′-disalicylidene)-formyhydrazone (H4L), two binary cocrystals H4L·H2O and H4L·2CH3OH, and four bimetallic nanostructures, namely, molecule cleft [Cu2(L)(Im)2]·(CH3)2CO (1) and squares [Fe2(H2L)2](ClO4)·2H2O (2), [Fe2(H2L)2]·4H2O (3), and [Mn2(L)(H2L)]·4H2O (4) (Im = imidazole) were synthesized and characterized. H4L with plentiful chemical information, such as allosteric interaction, ketone and enol isomers, potential axial chirality and multiple binding sites, and valent states can be recognized in assemblies. In cocrystals H4L·H2O and H4L·2CH3OH, the conformations of H4L altered by solvent-assisted interactions in turn affect the hydrogen-bonding assembly of (R)-H4L and (S)-H4L molecules in solid. The architectures of bimetallic cleft 1 and squares 2−4 are mainly determined by the cooperation of the metal’s coordination modes and the conformations of H4L. Complexes 1−4 contain discrete chiral dimeric units, further extending to multidimensional racemic hydrogen-bonding networks, in which the racemic mixture of H4L can be partially separated through metal-induced homochiral recognition. Very interestingly, the synthesis reactions for squares 2−4 were accompanied with spontaneous redox reactions where ferric ions were half and completely reduced to Fe(II) in 2 and 3, respectively, and Mn(II) ions were half-oxidized to Mn(IV) in 4. Presumably, the electronic delocalization upon the backbone of the ligand reflected by structures and theoretical calculations triggers the redox activity. Ligand H4L and complexes 1−4 display multiple strong absorptions in the UV−vis regions. Theoretically, the LE bond of free ligand H4L can be attributed to a predominant phenol → carbonyl intraligand character with a minor phenol → pyridine ILCT contribution while the HE peak has a multitransition character. The spectrum of 1 features the intense ILCT and LLCT π → π* absorptions, together with intense n → π*, very similar to that for free ligand H4L. In contrast, the spectra of 2−4 are red-shifted obviously and those electronic absorption spectra exhibit a common character of metal to ligand (MLCT) d (M) → π* (L) transitions.